In general, as semiconductor devices or integrated circuits have become smaller, the corresponding current densities across the metal conductors in the devices have increased. Metal conductors typically have an upper current density limit imposed by the phenomenon of electromigration. For example, aluminum conductors experience electromigration problems at current densities of approximately 10.sup.5 amperes per square centimeter (A/cm.sup.2).
Electromigration refers to the transport of mass in metals due to electric current. Electromigration is caused by the transfer of momentum from the electrons associated with the electric current to the positive metal ions. When a significant amount of current passes through thin metal conductors in semiconductor devices or integrated circuits, the metal ions associated with the thin metal conductors are transported and tend to accumulate in some regions and be removed from other regions. The accumulation or pileup of the metal ions can short circuit adjacent conductors in the device. The removal of metal ions in other regions may create voids which result in an open circuit. Short circuits and open circuits caused by electromigration often result in device failure.
Electromigration is a particular problem in multilayer semiconductor devices which include conductive vias or contacts connecting at least two conducting lines or paths. Conductive lines in semiconductor devices are generally thin layers (e.g., approximately 4,000 Angstroms (.ANG.) thick) of aluminum (Al) or an alloy of Al. The excessive current density associated with the conductive vias cause electromigration problems on the conductive line. The area proximate the conductive via commonly experiences excessive current densities because current from two or more conductive paths generally merges at the conductive via. Generally, a void is formed on one side of the via and a pile-up is formed on the other side of the via in accordance with the direction of current flow through the via.
Typically, the Al conductive lines are doped with copper (Cu) (e.g., alloying the aluminum with 0.5%-2% of Cu). Al conductive lines are generally doped with less than 2% Cu because higher concentrations of Cu make the lines difficult to dry etch. The Cu in the Al conductive line makes the conductive line less susceptible to the harmful effects of electromigration due to the higher electromigration resistance of the Cu. However, even conductive lines containing approximately 1% Cu or more are susceptible to electromigration as semiconductor devices become smaller and consequently, current densities increase.
Thus, there is a need for a multilayer semiconductor structure which is less susceptible to the harmful effects of electromigration. There is also a need for a conductive via which does not create electromigration problems on the conductive line. There is further a need for semiconductor structure having lightly doped (less than 2% Cu) Al conductive line which is less susceptible to electromigration problems proximate the via.